Pinkhill Meadow is located in a 4 hectare meander of the River
Thames at Farmoor Reservoir. In a detailed landscape assessment
of the reservoir site in 1988 the meadow was identified for its
potential for wetland creation. Few areas of wetland had survived
agricultural improvement in this part of the Upper Thames Valley.
Approximately 1.5 hectares of the meadow still had a valuable
relic meadow flora including species such as Adders Tongue,
Great Burnett and Pepper-Saxifrage.
The aim of the scheme was to complement the river and the reservoir habitats by restoring the floodplain wetland within the meadow which had been largely disturbed during reservoir construction.
A key objective was to restore habitat for breeding waders and wildfowl, notably redshank, and to create a place where people could experience a wide variety of wetland wildlife at close quarters and enjoy a more "natural" floodplain landscape. The project was a collaboration between the landowners ( Thames Water) and the National Rivers Authority (NRA), advised by Pond Action. A concept plan for the site was prepared from the landscape appraisal and developed into the detailed design, incorporating the project objectives with the site constraints.
The mosaic of over 40 ponds and pools was designed to maximise the topographical, and hydrological diversity of the site. This included specific creation of individual waterbodies with a wide range of maximum depths and permeability, and low angle, undulating drawdown zone areas to encourage wetland plant diversity.
Two phases of excavation were undertaken, the first in June and July 1990 and the second over the winter period in 1991/2. By phasing the works it was possible to better understand the detail needed for the more complex works in the second phase. Excavation was based on detailed landscape design drawings provded by the NRA landscape architect, and firmly led by close project management and continuous on-site supervision from key members of the project team. In this way the inexperienced machine operator was able to achieve the very subtle variations in topography in relation to water levels. The 20,000m³ of excavated material was carefully graded into a low hill near the adjacent Pinkhill Lock, but outside of the floodplain. This was then planted with trees and shrubs and sown with a wildflower seed mix.
In phase 1 four waterbodies were created; the main pond, wader scrape and two reedbed pools. In phase 2 the existing waterbodies were extended, added to and re-profiled to create areas of shallow water, wet meadow, mudflat and temporary pool habitat. Observations of the phase 1 works provided valuable detail for the improvements undertaken in phase 2. Observations of actual as opposed to design water levels in the pools were used to refine the new excavation levels, marginal areas and undulating contours of the wet meadow. The location of the mudflats was also based on the usage of the various areas of the site by different bird species.
Key features created:
The main pond is up to 2.5m deep and covers an area of just under 0.5ha and was excavated down into the gravel aquifer. The size and depth increases the diversity of habitats and isolates the several islands reducing the likelihood of predation of bird nests. The depth also ensures open water and from a management viewpoint it also restricts the complete colonisation by marginal wetland plants.
Shallow-water areas and edges
These areas were designed to between 0.3m below and 0.1m above normal water levels. As the main pond level will fluctuate by about 0.3m, reflecting groundwater levels, these areas are important to retain shallow slopes at the water's edge.
Temporary ponds and pools
The site also includes small temporary pools, some isolated and some bordering the larger waterbodies. Sizes vary from a few square metres to the larger two semi-permanent ponds (approximately 100m²). These transient ponds are designed to dry out in drought years (two or three times since excavation) and provide a habitat with low fish predation, benefiting many aquatic invertebrates and some amphibians.
A 400mm maximum depth shallow pool was formed within the alluvium overlaying the gravel aquifer and the water level controlled by means of a connecting pipe to the main pond. This feature provides extensive muddy margins for feeding waders and Teal, particularly during autumn migration.
Gravel islands and margins
Created over an area of 0.1ha, these provide nesting habitat for Little Ringed Plover and Common Tern. The gravel was carefully selected from a local source to ensure a good size distribution, important for some nesting birds. Selectively placed cobbles and boulders also provide some cover.
Mudflats and islands
These were created by excavating into the alluvium. Gentle slopes of 1:20 minimum provide feeding and nesting habitats for wading wildfowl, but also created a more open habitat suitable for some marginal wetland plants.
Undulating wet grassland
Small variations in topography were engineered to create an undulating meadow with water levels close to the surface (between 0.1m and 0.2m above normal water level). This marshy/tussocky Rush and Sedge dominated area was designed to provide feeding and nesting areas for waders, particularly Redshank and Snipe.
Two linear reedbeds, totalling over 250m in length, were excavated along the eastern edge of the site. Shallow trenches were dug and planted with pot grown Common Reed. These serve as a boundary to human disturbance from adjacent footpaths and provide valuable nesting and foraging habitat for wetland birds such as Reed Warbler and Water Rail.
A double row of mixed shrubby Willow (incorporating some Hawthorn, Blackthorn and Dog Rose) forms a 4m wide hedge at the eastern edge of the site linking adjacent areas of reed, meadow, hedges and woodland. As with the reedbed the presence of the hedge also helps to mitigate the disturbance of the nearby footpath, as well as sustaining a rich insect population and providing over-wintering cover for frogs, toads and newts. The scrubby character is to be retained by staggered coppicing on an annual basis ensuring a permanent screen is maintained.
Continuous post-project appraisal was carried out on this site for the first 5 years after construction and the results showed that this small wetland creation scheme quickly acquired an extremely rich wildlife community.
In these 5 years over 20% (over 60 species) of all Britain's wetland and aquatic plant species colonised the site. In the main pond alone the plant community was one of the richest recorded in ponds in the county. Similarly 22% (over 150 species) of Britain's macroinvertebrate species were recorded on the site, including 12 breeding species of dragonfly.
Breeding wader densities have been very high, in one year up to 100 pairs/km² equaling that of grazing marsh and other important British wader habitats. In 1993 and 1994 two pairs of Little Ringed Plover bred, representing 15% of Oxfordshire's breeding population. Unfortunately the site was too small to sustain such densities and the plovers have not returned since 2000.
As a result of the commitment of the partners and the continued appraisal of the site's development some minor and major modifications have been funded in every second or third year between 1992 and 2002. These have included: managing gravel islands; scraping new mudflats; creating new pools; doubling the size of the main reedbed; annual coppicing and thinning of the willow scrub.
A key reason for the huge success of Pinkhill, in terms of its pond creation, is the combination of three critical factors for creating biologically high quality sites: good water quality; high degree of landscape connectivity to other wetlands; complex mosaic design.
Original Information Providers: Richard Hellier Ponds Conservation Trust
The Pinkhill Meadows project has been extremely successful in demonstrating that high biodiversity clean water ponds can be created. The concepts at Pinkhill have been applied extensively in the Million Ponds Project and in UK biodiversity policy. Four monitoring pools have been regularly surveyed since works were completed. These have indicated that rich macrophyte, aquatic macroinvertebrate and wetland bird assemblages have been created, with rapid colonisation following project works. Following colonisation, the site is now in the top 10% of pond sites in the UK for aquatic macro invertebrates, supporting approximately 20% of all UK wetland plant and macroinvertebrate species.
Removal of Bulrush ( Typha latifolia ) occurred in the 5 years following the works to prevent initial domination by this species. Invasive species, in particular New Zealand Pygmyweed (Crassula helmsii ), are a recent concern. Management of the site has changed over time - sometimes left abandoned for a time and, at others, focussed on specifically. Costs for this have been modest with a lot of the work carried out by volunteers. The reintroduction of grazing in 2008 has been welcomed as a management technique to open up marginal areas and prevent further encroachment of reeds. Maintaining grazing is identified as a priority.